1. Field of the Invention
The present invention relates generally to a closed vessel liquid reagent dispensing assembly of the type in which liquid is dispensed from a dip-tube liquid discharge conduit from a gas-pressurized vessel, and in which the liquid level may be sensed by a sensor extending downwardly in the vessel and terminating just short of the floor thereof.
2. Description of the Related Art
In the field of chemical vapor deposition using liquid delivery systems for the supply of a liquid reagent to a vaporizer, wherein the liquid is vaporized by the vaporizer and passed to a chemical vapor deposition reactor for forming the desired material layer on a substrate from the reagent vapor, the liquid reagents employed are often costly.
This is particularly true in instances where the material sought to be deposited in the chemical vapor deposition operation is a precious metal or rare element, or where the liquid reagent is difficult and/or expensive itself to synthesize. Examples include many liquid reagents for liquid delivery chemical vapor deposition manufacturing of semiconductor and superconductor devices and precursor structures thereof, e.g., liquid source compounds, and solutions of solubilizable or suspendable source compounds, for platinum, gold, titanium, lead, palladium, zirconium, bismuth, strontium, barium, calcium, antimony, thallium, tantalum, etc.
Owing to the costly character of such liquid reagents, it is obviously desirable to maximize the utilization of the reagent in a given application, and to minimize wastage or loss of the valuable liquid reagent material.
The conventional supply vessel assembly used in liquid delivery chemical vapor deposition operations typically comprises a closed vessel including floor and wall members defining an interior volume, bounded by interior floor and wall surfaces, for holding liquid reagent.
The vessel is coupled with pressurized gas supply means for introducing pressurized gas into the interior volume of the vessel, to thereby pressurizingly dispense liquid reagent from the vessel. For such purpose, the vessel may be provided with a "pressurization post," or inlet tube, which is joined by a conduit to a source of pressurized gas, such as a high pressure gas cylinder containing a displacement gas such as nitrogen, argon, or other suitable gas species, which serves to impose pressure on the volume of liquid reagent in the vessel, to cause the egress of liquid from the vessel under dispensing conditions.
Liquid is discharged from such vessel by a "dip tube," or liquid discharge tube, which is arranged to extend upwardly and exteriorly of the vessel, from a lower open end in close proximity to the flat or generally planar floor surface of the vessel's interior volume. The aforementioned pressurization of the liquid reagent under dispensing conditions thereby causes the liquid reagent to flow into the open lower end of the dip tube and upwardly therethrough for discharge exteriorly of the vessel, e.g., into a line feeding the vaporizer for the chemical vapor deposition unit, whereby reagent source vapor is formed and subsequently introduced to the chemical vapor deposition reactor for the deposition of a material layer on the substrate therein.
The conventional liquid reagent vessel of the above-described character also is equipped with a liquid level sensor, of a type extending downwardly in the interior volume and terminating at a lower end in close proximity to the floor surface in the interior volume of the vessel. The sensor is designed to detect the level of the liquid as it is being depleted.
The liquid level sensor and the dip tube need a certain height of liquid to function properly. If the liquid level is too low, then the level sensor can cease to function and more importantly, gas can be pressurized up the dip tube. The passage of gas up the dip tube is undesirable in liquid delivery applications, since the "bypassing" gas replaces the liquid sought to be vaporized in the egressing fluid. As a result, the vaporizer may overheat or develop hot spots, and the downstream process may be starved of the desired flux or flow rate of source vapor, which in turn can cause the deposited material to be stoichiometrically or otherwise compositionally deficient or even useless for its intended purpose.
To overcome this problem, it has been conventional practice to utilize only 80-90% of the liquid reagent volume originally provided in the vessel, meaning that 10 to 20% of the original volume of reagent liquid is not used (and therefore wasted), to ensure that the liquid level is high enough for liquid level sensing at all times. Because the semiconductor manufacturing process operates in a batch process fashion, with respect to the deposition of constituent materials on the wafer substrate from the vaporized source material, the non-used reagent from the supply vessel becomes part of the overall waste from the semiconductor manufacturing plant.
In instances where the liquid reagent is costly and valuable, such wastage of the liquid reagent severely adversely impacts the process economics, as well as representing a significant burden in terms of disposition of the waste liquid and its environmental impact.
Accordingly, it would be a significant advance in the art, and is accordingly an object of the present invention, to provide an improved liquid reagent supply means and method which increases the usage of the liquid reagent, and correspondingly reduces waste thereof, relative to liquid reagent supply means and method of the prior art.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.